This invention relates to a surface acoustic wave filter.
Surface acoustic waves are effectively excited by an interdigital transducer constructed on a surface of a piezoelectric substrate within frequency range determined by the structure of the interdigital transducer and surface acoustic velocity of the piezoelectric substrate. SAW (surface acoustic wave) filters were proposed utilizing the properties in which a first transducer converts an input electrical signal to a surface acoustic wave with frequency characteristics determined by the structure of the transducer, and a second transducer receives the acoustic wave to convert the surface acoustic wave to an output electrical signal with frequency characteristics determined by the structure of the transducer. Filters for many objects, for example a video-interfrequency-filter (VIF) of a television set, are expected.
However, the fact is that SAW filters are not widely used because it is difficult to obtain low insertion loss without ripple within pass band by using SAW filters. The ripple of the amplitude of a transferred signal and also the ripple of group delay are caused by multi-reflection of the surface acoustic wave between the input and the output transducers. Twice-reflected waves are largest among the multi-reflected waves. This wave is called "Triple Transit Echo" or "TTE". That is, surface acoustic wave launched by the input transducer propagates toward the output transducer and the incident wave to the output transducer is reflected backward and propagates toward the input transducer, and further the wave is again reflected by the input transducer toward the output transducer. The twice-reflected acoustic wave (triple transit wave) delays by 2.tau. in time when measured with the main wave (once-transit wave) where .tau. is a time for the acoustic wave to propagate from the input transducer to the output transducer and this causes ripple in amplitude and group delay response by the interaction of the twice-reflected wave and the main wave.
In common SAW filters, it is known that the level of the TTE is expressed approximately by an equation: EQU TTE(dB).apprxeq.-(2.times.Insertion loss+6)(dB).
For example, the level of TTE becomes -26 dB if the limit of the insertion loss for practical usage is determined to be 10 dB. The level -26 dB is not acceptable. For practical usage, about -40 dB or less than that is required. Such high level of the TTE as mentioned above causes a ghost or picture color distortion such that the desired color is not reproduced, and further the picture is blurred or becomes conversely an enhanced sharp picture.
There are two reasons for the multi-reflection in SAW filters as described above. One is discontinuity of acoustic impedance caused by the metal deposited on piezoelectric substrate to make input and output transducers. The other reason is regeneration of an acoustic surface wave by the output transducer, in which the transducer receives the incident wave launched by the input transducer and also acts as a transmitter. Then amplitude and phase of the regenerated wave depend on the load connected to the output transducer. Such regeneration of the acoustic wave also occurs in the input transducer. Then, multi-reflection occurs. Multi-reflection caused by the latter reason is difficult to remove. It is known that multi-reflection caused by the former reason can be easily removed by using split electrodes in which each electrode (or finger) of the interdigital transducer has a width of one eighth of the wavelength of the surface acoustic wave at a frequency at which the transducer has maximum response and also has spaces of one eighth of such wavelength. However, the multi-reflection caused by the latter reason is difficult to remove.
Some methods to remove the multi-reflection caused by the latter reason have been proposed. One example is a method to use extra reflection transducers other than input and output transducers, in which the reflection transducers are placed apart from the input or output transducer by one fourth of such wavelength, and the waves reflected by the reflection transducers interact with the wave reflected by the input ot output transducer to eliminate the wave reflected by the input or output transducer. Another example is a method to use an input transducer which is driven by two or three drivers (sources) with phases different from each other by 120.degree., and to obtain a surface acoustic wave propagating in one direction, and further to use an output transducer which is similar to the input transducer and which acts as an inverse converter for converting acoustic waves to electrical signals. However, these methods for eliminating the TTE have disadvantages. For the former case, a large piezoelectric substrate is required to deposit the reflection transducer. For the latter case, the structure of the input and output transducers become complex to require drivers (sources) with two or three different phases and also circuits to inversely convert the signals.